gh-104050: Argument clinic: more misc typing improvements

[AlexWaygood]

• Issue: Add type annotations to clinic.py #104050

[AlexWaygood]

Mypy was complaining because our type annotation for add_c_converter says (correctly) that the argument passed in has to be a subclass of CConverter. But that isn't necessarily true here, since this metaclass could theoretically be used with classes other than CConverter (it isn't, and it won't be, but mypy doesn't know that).

Ideally we could do this:

    def __init__(
        cls, name: str, bases: tuple[type, ...], classdict: dict[str, Any]
    ) -> None:
        assert issubclass(cls, CConverter)
        add_c_converter(cls)
        add_default_legacy_c_converter(cls)

But we can't... because if cls is CConverter itself, then at this point, CConverter doesn't exist in the global namespace yet! So that would fail with NameError.

I therefore elected to use typing.cast() to just tell mypy to go away here.

[erlend-aasland]

Thanks for writing these excellent explanations!

[AlexWaygood]

I'm glad they're all making sense! :D